Various machines, such as a gas turbine and/or a steam turbine, may be used to drive a load such as a power generator. In particular, a gas turbine and/or a steam turbine may be used to rotate a magnet within a stator to generate electric power. The power generator includes a shaft which is connected to the rotating magnet and which itself is connected to a large connecting shaft (also called a load coupling shaft) rotated by one or more turbines. The connecting shaft is typically large and stiff, thereby resulting in very small torsional displacements (strains) when a torque is imposed on the connecting shaft. A measurement of torque transmitted through the connecting shaft is often made to determine the power output of the machine(s) rotating the connecting shaft.
The torque imposed on the connecting shaft has been measured in the past using strain gauges. However, the accuracy of torque measurements provided by strain gauges often does not meet engineering requirements because the uncertainty of such measurements is rather large as compared to the strains measured.
A known digital light probe system, developed by GE Aircraft Engines, has been used for several applications in the past including measuring compressor rotating blade vibratory displacements.
An existing high resolution torque measurement method employs a pair of high-powered laser probes, one probe at each end of the measured shaft. The probes are perpendicular to the shaft and their tips are positioned at roughly 0.050″ from the outer surface of the shaft. A pair of targets, one at each end, are placed between the probes and the shaft at corresponding locations and bonded to the surface of the shaft. As the targets pass by the probes in every revolution when the shaft is rotating, the timing measurement system records all the time of arrival data for the two probes. The software within the measurement system then processes the data by comparing the timing difference between the probes at two ends and convert the results into torsional displacement. Once the torsional displacement is known, the torque on the shaft is calculated using an analytical model or formula.
The method described above works when the shaft remains stationary radially during rotation. However, the measured value may include errors produced from shaft movement when the shaft is subject to radial movement at either end and the torque value is subject to misinterpretation. Accordingly, there remains a need in the art for an improved apparatus and method to measure torque on a rotating shaft, such as a rotating load coupling shaft for driving a power generator, with a high degree of accuracy using multiple probes to remove errors produced from shaft movement so that the measured value represents the true torsional displacement of the shaft.